This invention relates to the continuous melting of a metallic charge to form a molten steel product. The process is particularly advantageous in those regions where there is a concentration of production of, or ready availability of scrap and/or direct reduced iron (DRI), and where electric energy is both available and economical.
Heretofore, the operation of an electric arc steelmaking furnace has been an intermittent operation, wherein the sequence followed is: charging of steel scrap and/or direct reduced iron, pig iron, slag formers and alloying elements; ignition or establishment of an electric arc between the electrodes in the furnace to create melting conditions for melting the charge and forming a molten metal bath covered by a molten slag; refining for a period of time during which the molten metal portion of the bath is refined to form steel having a desired composition and quality; and periodically raising the electrodes to remove them from contact with the bath and interference with the tapping procedure; and then tapping the molten metal. In addition, slag can be removed by a slagging, or slag-off, operation as required.
Although this invention is shown and described in connection with an electric arc steelmaking furnace, it will be readily apparent that any electric powered steelmaking furnace including but without limitation, plasma furnaces and induction furnaces could be substituted for the electric arc steelmaking furnace with like results.
There is currently a steelmaking practice known as "continuous charging" or "continuous melting", but these practices refer to a charging practice in which charge materials are fed to a furnace during the charging, melting and refining periods, then charging is interrupted and power input is interrupted for the tapping procedure. It has been found that an electric steelmaking furnace can be operated continuously without interruption of charging or power input for the tapping procedure by the taking the following steps in the steelmaking process.
First, scrap must be prepared by shredding or shearing it to a suitable size. The scrap is preferably segregated for quality control. As received, the scrap is segregated into desired classifications, preferably depending on contamination by tramp elements sulphur and phosphorus. Segregated scrap is shredded or sheared and stored for use. By maintaining a stock or shredded or sheared raw material, continuous operation of the process is assured during periods of shredder or shear down-time.
Direct reduced iron is normally prepared in the form of lumps or pellets, which are generally of a size of less than about one half inch diameter. Direct reduced iron briquets can also be used as feed material. Preferably such direct reduced iron is produced at a contiguous plant.
Scrap, direct reduced iron, slag formers and alloying materials are preheated and continuously fed to the electric arc furnace. A foaming slag practice is used, and the furnace is only partially tapped intermittently without removal of the electrodes, thus electrodes remain at full power during both continuous feeding, refining (which is continuous) and tapping (which is intermittent). Tapping is carried out by limited tilting of the furnace, generally not varying more than 15.degree. from the vertical.